The present invention relates to a process for purifying methacrylamide by the recrystallization from an aqueous medium.
For the preparation of methacrylamide, various processes have been known. Among them, the most inexpensive and easy process comprises neutralizing methacrylamide sulfate obtained from aceton cyanhydrin and sulfuric acid. This process has been disclosed in the specifications of Japanese Patent Publications Nos. 26094/69, 2566/70 and 35885/72.
Though methacrylamide prepared by the above process is sufficiently usuable as a fiber modifier, paper-converting agent or starting material of an emulsion, it cannot be used as it is mainly as a resin modifier for a photosensitive resin, windshield, photographic film or coagulant the demand of which has been increased recently.
A reason therefor is that a quite high transparency is required of the above resin products and, therefore, the contamination of the resin with even only a very small amount of impurities which reduces the transparency must be avoided as far as possible. In other words, an extremely high transparency such as no turbidity or a high see-through of a solution of methacrylamide used as the modifier is required. If such impurities are incorporated therein even in a very small amount, the crosslinking reaction or polymerization reaction of the resin to be modified is significantly inhibited in many cases.
Transparency of the solution is an important criterion of the quality to know whether the solution is useful for said purpose or not. It has been said from experiences that a see-through described below of at least 50.degree. is required for this purpose. However, methacrylamide available on the market does not satisfy the condition of transparency such as the above see-through at all.
The most simple and practical process for the purification of methacrylamide comprises recrystallizing methacrylamide from water or ammonia liquor (see, for example, the specification of Japanese Patent Publication No. 35885/1972). However, according to the inventors' experiments, when methacrylamide is recrystallized by the above general process, the most important transparency of the solution is hardly improved and, therefore, it cannot be used in the fields in which the above high quality is necessitated, though purity and chromaticity of methacrylamide are improved by the recrystallization.
As the impurities which contaminate methacrylamide, there may be mentioned various, inorganic and organic substances. Among those impurities, inorganic salts such as ammonium sulfate formed in the course of the neutralization can be removed relatively easily by an ordinary recrystallization process. However, it is quite difficult to remove polymers which are the main cause of the reduction of transparency. Reasons therefor are as follows: The polymers contained in the aqueous solution are too fine to be filtered and, therefore, incorporated again in methacrylamide recrystallized. Further, the polymers are water-soluble and, therefore, a great part thereof is crystallized out in the recrystallization of methacrylamide. Accordingly, the polymers cannot be removed completely by the recrystallization from water.